Field of the Invention
This invention relates to an improvement in the liquid encapsulated Czochralski crystal pulling technique of forming highly conductive semiconductor crystals.
It is known that high dopant levels of group IVB or group VIB elements are difficult to achieve in a group IIIB-VB semiconductor crystal grown by the liquid encapsulation Czochralski technique (LEC). As a result, carrier concentrations above 3.times.10.sup.17 per cubic centimeter are not common. One reason for this problem is that the boric oxide used as the barrier or encapsulating liquid is highly hygroscopic and cannot be maintained absolutely moisture free. When used as an encapsulant for growing doped semiconductor crystals, the dopant acts as a gettering substance and hence some of the dopant forms an oxide rather than doping the semiconductor crystal as the crystal is grown. One method used to increase the amount of dopant in the melt, is to add a greater excess of the dopant. While this may increase the dopant concentration in the final crystal, it is difficult to achieve uniform concentrations within the crystal, and difficult to control exactly how much dopant will end up in the crystal.
Other techniques for increasing the dopant level include pre-drying the boric oxide with or without a gettering substance, and the use of magnetic fields. The prior art usually requires changes in equipment or additional processing steps to achieve the desired results.